The present invention relates to lithium secondary cells comprising a positive electrode, a negative electrode and a nonaqueous electrolyte, and more particularly to improvements in active substances which undergo a reversible electrochemical reaction with lithium ions for use in the positive electrode or negative electrode of such a lithium secondary cell.
For use in portable electronic devices such as compact video cameras, portable telephones and notebook personal computers, attention has been directed in recent years to lithium secondary cells, typical of which are lithium ion cells wherein the positive electrode is prepared from a lithium-containing transition metal oxide such as lithium-cobalt oxide (LiCoO2), lithium-nickel oxide (LiNiO2) or lithium-manganese oxide (LiMn2O4), or manganese dioxide (MnO2), and the negative electrode active substance used is metallic lithium, lithium alloy, carbon material capable of absorbing and desorbing lithium ions, or the like. Among these cells, lithium secondary cells have already been placed into actual use in which the carbon material serves as the negative electrode active substance.
Various materials of active substances other than those mentioned above are proposed for use in lithium cells. For example, U.S. Pat. No. 4,223,079 proposes lithium cells wherein a metal sulfide is used among these materials. Stannous sulfide (SnS) is used as the metal in the proposed lithium cell in an attempt to give improved characteristics to the cell.
However, the metal sulfide used in the proposed lithium cell as an active substance, i.e., stannous sulfide (SnS), is unstable in crystal structure and therefore has the problem that the cell can not be charged and discharged repeatedly and is consequently unusable as a secondary cell because of poor cycle characteristics.
An object of the present invention is to stabilize the crystal structure of stannous sulfide (SnS) and provide a lithium secondary cell exhibiting practically useful charge-discharge characteristics.
The present invention provides a lithium secondary cell wherein a compound sulfide having substantially the same crystal structure as stannous sulfide (SnS) or a lithium-containing compound sulfide comprising the compound sulfide and lithium contained therein is used as an active substance of one of the positive electrode and the negative electrode of the cell. The compound sulfide comprises at least one transition metal selected from among copper (Cu), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co) and nickel (Ni), tin and sulfur. The compound sulfide is represented by the formula MxSn1xe2x88x92xS wherein M is at least one metal selected from among Cu, V, Cr, Mn, Fe, Co and Ni, and 0.02xe2x89xa6xxe2x89xa60.5.
The compound sulfide having substantially the same crystal structure as stannous sulfide (SnS) or the lithium-containing compound sulfide comprising this compound sulfide and lithium contained therein contains the metal element M (Cu, V, Cr, Mn, Fe, Co or Ni) in the crystal lattice of stannous sulfide (SnS), and the crystal structure of stannous sulfide (SnS) is stabilized with the Sn partly replaced by the metal element M. Accordingly, when the compound sulfide or the lithium-containing compound sulfide is used as an active substance for the positive electrode or negative electrode in fabricating a lithium secondary cell, the cell can be given improved charge-discharge cycle life characteristics. Incidentally, X-ray diffraction, for example, reveals that the compound sulfide has substantially the same crystal structure as stannous sulfide (SnS).
It is desired that the metal element M be selected from among copper (Cu), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co) and nickel (Ni) which have been found effective for affording improved charge-discharge cycle life characteristics. It is known that these metal elements M each form a stable compound having a decomposition temperature of at least 1000xc2x0 C. when combined with sulfur (S) [see, for example, Binary Alloy Phase Diagrams (1986), American Society for Metals, Mxe2x80x94S Binary Phase Diagram]. Stated more specifically, a great chemical bond force acts between the metal element M and sulfur (S), causing the element M to occupy part of the crystal lattice of stannous sulfide (SnS) phase to stabilize the crystal structure. This affords improved charge-discharge cycle life characteristics to the lithium secondary cell wherein the compound sulfide or the lithium-containing compound sulfide is used as an active substance.
Other metal elements M, such as cadmium (Cd), indium (In), molybdenum (Mo), lanthanun (La), cerium (Ce), samarium (Sm) and platinum (Pt), which form a compound with sulfur (S) are also expected to be similarly effective for giving improved charge-discharge cycle life characteristics. The upper limit for the proportion x of the metal element M in the compound sulfide is 0.5 because if the proportion x of the metal element M exceeds 0.5, the single phase of metal element M or the sulfide phase will precipitate to impair the effect to improve the cycle life characteristics. In other words, limiting the proportion x of the metal element M to not higher than 0.5 prevents the precipitation of the single phase of metal element M or the sulfide phase to result in a stabilized crystal structure and improved cycle life characteristics.
In the case where the compound sulfide having substantially the same crystal structure as stannous sulfide (SnS) or the lithium-containing compound sulfide is used as a positive electrode active substance, preferably as the negative electrode active substance to be used is a carbon material, such as graphite (natural graphite or artificial graphite), coke or organic fired body, which is capable of electrochemically absorbing and desorbing lithium (Li), Lixe2x80x94Al alloy, Lixe2x80x94In alloy, Lixe2x80x94Alxe2x80x94Mn alloy or like lithium alloy, or metallic lithium. In this case, the final charge voltage is about 3.4 V, and discharge voltage is about 2.9 V.
Among these negative electrode active substances, the carbon material, when used, affords a greater effect to improve the cycle life characteristics. This is attributable to the fact that the carbon material, unlike lithium alloys or metallic lithium, will not grow into dentritic crystals due to charging and discharging and leading to short-circuiting. Additionally, the carbon material is free of the likelihood that sulfur (S) dissolving in a very small amount in the electrolyte reacts with the lithium (Li) in the negative electrode of lithium alloy or metallic lithium to form on the negative electrode surface a compound such as Li2S [see, for example, Binary Alloy Phase Diagrams, Vol. 2, p. 1500(1986), American Society for Metals, Lixe2x80x94S Binary Phase Diagram] which will cause inactivation.
Further in the case where the compound sulfide having substantially the same crystal structure as stannous sulfide (SnS) or the lithium-containing compound sulfide is used as the negative electrode active substance, preferable as the positive electrode active substance is a lithium-containing transition metal oxide such as LiCoO2, LiNiO2, LiMn2O4, LiMnO2, lithium-containing MnO2, LiCo0.5Ni0.5O2 or LiNi0.7Co0.2Mn0.1O2. Available in this case is a lithium secondary cell which is about 1.2 V in final charge voltage and about 0.7 V in discharge voltage. When the compound sulfide having substantially the same crystal structure as stannous sulfide (SnS) or the lithium-containing compound sulfide is used for the negative electrode, more improved cycle life characteristics result than when the sulfide is used for the positive electrode. This is attributable to a lower charge voltage which suppresses decomposition of the electrolyte.
The compound sulfide having substantially the same crystal structure as stannous sulfide (SnS) can be prepared by firing a mixture of the elements constituting the compound sulfide, compounds of the component elements or a mixture of these compounds. Preferably the firing temperature is at least 231xc2x0 C. to not higher than 880xc2x0 C. As will be apparent from an Snxe2x80x94S binary phase diagram [see, for example, Binary Alloy Phase Diagrams, Vol. 2, p. 2004(1986), American Society for Metals], the reason is that temperatures higher than 880xc2x0 C. are likely to produce a fired body as melted, with the result that when cooled to room temperature, the body becomes uneven in composition, failing to give a sufficient effect to improve the cycle characteristics. Further temperatures lower than 231xc2x0 C. permit stannous sulfide (SnS) to remain stable, failing to fully diffuse the metal element M added through the crystal structure of stannous sulfide (SnS) and produce a compound sulfide.
The solvent for the electrolyte for use in the lithium secondary cell of the invention is preferably a solvent mixture of a cyclic carbonate, such as ethylene carbonate, propylene carbonate or butylene carbonate, and a chain carbonate such as dimethyl carbonate, methylethyl carbonate or diethyl carbonate. Also preferable is a solvent mixture of a cyclic carbonate and an ether solvent, such as 1,2-dimethoxyethane or 1,2-diethoxyethane.
The solute to be used is preferably LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2) (C4F9SO2), LiC(CF3SO2)3, LiC(C2F5SO2)3 or the like, or a mixture of such compounds. Further usable are a gel polymer electrolyte which is prepared by converting a liquid electrolyte to a gel with a polymer such as polyethylene oxide or polyacrylonitrile, and an inorganic solid electrolyte such as LiI or Li3N. The electrolytes for use in the lithium secondary cell of the invention are not limited to those mentioned above. Further usable without limitations are those comprising a lithium compound serving as a solute having ionic conductivity and a solvent for dissolving or holding the solute therein insofar as such electrolytes are not decomposed by voltage during charging, discharging or preservation.